a) Field of the Invention
This invention relates to thermoelectric semiconductors useful, for example, in a thermoelectric cooling device or an electricity generating device and also to a fabrication process thereof. In particular, the present invention is concerned with thermoelectric semiconductors formed of sintered compacts and also with a fabrication process of the thermoelectric semiconductors.
b) Description of the Related Art
For use as thermoelectric semiconductors in a thermoelectric cooling device or the like, a Bi.sub.2 Te.sub.3 system material grown in the form of a crystal is machined and processed. These machining and processing procedures include slicing a Bi.sub.2 Te.sub.3 system, thermoelectric semiconductor crystal in thicknesses of that of an intended device to obtain wafer-shaped thermoelectric semiconductors and then etching surfaces of the wafer-shaped thermoelectric semiconductors. This etching treatment is applied to remove finely-cracked surface layers formed upon slicing and processing-modified layers of oxides or the like and also to obtain sufficient bonding strength with associated Ni-plating layers to be described subsequently herein. Each wafer-shaped thermoelectric semiconductor is diced into individual semiconductor chips. Each semiconductor chip is then joined to electrodes with a low melt solder such as a Pb--Sn eutectic alloy.
As another process, each wafer-shaped thermoelectric semiconductor of the above-described type is roughened at surfaces thereof by treatment such as sandblasting, and Ni powder is thermally sprayed onto the surfaces. Subsequent to polishing the surfaces, the wafer-shaped thermoelectric semiconductor is diced into individual semiconductor chips, to which electrodes are then joined with a solder.
The former process has a low processing yield because a semiconductor crystal has low mechanical strength, and also requires a long time for the production of semiconductor crystals. The former process therefore leads to high fabrication cost. The latter process, on the other hand, is also accompanied by similar problems in mechanical strength and fabrication cost. In addition, a processing-modified layer is formed on each semiconductor layer and moreover, a thermally-sprayed Ni layer is not dense and oxides or impurities are contained in a joined interface, thereby resulting in a reduction in characteristics.
With a view to reducing the fabrication cost, fabrication processes of thermoelectric semiconductors by powder sintering are now under investigation. According to these processes, a powder mix of semiconductor raw materials is sintered into a block of 40 mm.times.30 mm.times.30 mm or so. The block is sliced in thicknesses of that of each intended semiconductor chip, whereby wafer-shaped thermoelectric semiconductors are obtained. Each wafer-shaped thermoelectric semiconductor is subjected at surfaces thereof to etching treatment and plating treatment and is then diced into individual semiconductor chips. Each semiconductor chip is joined to electrodes with a solder.
However, these conventional powder sintering processes are each accompanied by the drawback that sintered grain boundaries are damaged by etching treatment, the bonding strength available with electrodes is not so high as that available in the case of a crystal even if Ni-plating is applied, and the resultant thermoelectric semiconductor hence has low reliability.
In the meantime, still further processes have been proposed in each of which, after a wafer-shaped thermoelectric semiconductor is subjected at surfaces thereof to etching treatment and is machined and processed into semiconductor chips, each of the chips is directly joined to electrodes with a special Pb--Sn system solder [reference may be had, for example, to "Netsuden Henkan Shisutemu Gijyutsu Souran (Comprehensive Bibliography of Technologies on Thermoelectric Conversion Systems)", pages 24-28, The Realize Co., Ltd.]. A Pb--Sn system solder however involves the problem that its reaction with a semiconductor material tends to proceed during use and the characteristics of the semiconductor are progressively lowered in the course of use.